Homo- and heterobinuclear Cu2+ and Zn2+ complexes of abiotic cyclic hexaazapyridinocyclophanes as SOD mimics

Abstract

The new receptor 3,7,11,15,19,23-hexaaza-1(2,6)-pyridinacyclotetracosaphane (L1) containing a complete sequence of propylenic chains has been synthesised. The acid–base behaviour and Cu²⁺ and Zn²⁺ coordination have been analysed by potentiometric measurements in 0.15 M NaClO₄ for L1 and for the related compounds 3,7,11,14,18,22-hexaaza-1(2,6)-pyridinacyclotricosaphane (L2), 3,7,10,13,16,20-hexaaza-1(2,6)-pyridinacycloheneicosaphane (L3) and 3,7,10,12,15,19-hexaaza-1(2,6)-pyridinacycloicosaphane (L4). The crystal structure of [(CuH₄L2)(H₂O)(ClO₄)](ClO₄)₅·3H₂O shows an interesting combination of a metal ion coordinated by the pyridine nitrogen atom and the adjacent amine groups of the chain, and a perchlorate anion sitting at the middle of the site defined by the remaining four protonated ammonium groups of L1. Paramagnetic NMR data suggest that imidazole coordinates to the Cu²⁺ ions as a bridging ligand in a wide pH range. SOD activity for Cu²⁺–Cu²⁺ and Cu²⁺–Zn²⁺ complexes with L1–L4 have been measured by NBT assays at pH 7.4, obtaining some of the lowest values so far reported for SOD mimics. SOD activity has also been checked by chemiluminescence assays using polymorphonuclear leukocytes (PMNLs).